Laser light detectors have been available in the past for use in precisely determining the proper elevation on construction job sites. The standard method for using such detectors is to mount a rotating laser light source at a particular elevation on a construction job site, then mount the laser light detector on a piece of equipment (such as on the blade of a bulldozer) to let the operator of the equipment know precisely the elevation of the equipment while the it is in use. For example, the laser light detector could be mounted on a pole attached to the blade of a bulldozer, so the operator of the bulldozer could keep the blade at the correct position while grading the land to the precise elevation desired.
To be most effective, a laser light detector would have an easily viewable display that gives the elevation indication to a person who is sited a few feet from the detector. In addition, a detector used on a machine would normally have some type of photodiode or other photo-sensitive device on all four corners of the detector's enclosure, so that it could detect laser light coming from any direction. Typical laser light detectors must operate within a one hundred millisecond cycle time, since most rotating laser light sources rotate at 600 rpm.
Typical laser sources used as rotating laser light sources operate in either the infrared or red light frequency spectrum. For example, infrared laser diodes operating at 780 nm are commonly used, as well as red light helium-neon gas lasers, operating at 633 nm. The laser light is typically collimated. Various rotating laser light sources are available having beam sizes from as small as one-quarter inch in diameter to as large as three-quarter inches in diameter.
Laser light detectors are typically available in two types of models: a "machine control receiver" and a "hand-held receiver". The machine control receiver is typically mounted on a piece of equipment, such as a bulldozer, and used in the manner discussed above. The hand-held receiver is typically a smaller device which can be carried by a typical construction worker to be used to detect the elevation of locations at a moment's notice.
A typical hand-held receiver would have a single light-sensitive array having at least two photodiodes arranged in a vertical linear manner. As an alternative configuration, the photodiodes could be arranged in a split-cell geometry, as disclosed in U.S. Pat. No. 4,676,634 (by Petersen), or some other type of photodetector geometry. Examples of other photodetector geometries are provided in U.S. Pat. No. 4,907,874 (by Ake) which includes groups of interdigitated photodetector elements positioned adjacent to each other, by U.S. Pat. No. 4,976,538 (by Ake) which also uses interdigitated photodetector elements having a zig-zag shape to make the sensor less sensitive to being partially shaded from the laser light source. These patents primarily describe various photodetector geometries that are used to maximize the consistency of operation of the detector under varying conditions of laser spot size, image energy distribution, and overall power.
One problem of the conventional laser-light receivers is their use of inductors at the photodetector elements to limit noise from sunlight. The inductors are rather expensive electrical components and it would be desirous to eliminate them from a circuit design that could perform the same laser light detection function. Another problem of conventional peak detecting laser light detectors, as disclosed in the above-identified patents, is that as system noise increases, their signal-to-noise ratio decreases. In fact, if the noise increases substantially to the point where it swamps out the signal, then the signal-to-noise ratio becomes nearly equal to or less than 1.0. This problem is exacerbated when weather effects are added to the problem, which causes the signal-to-noise ratio to even decrease further.
Another problem in conventional hand-held receivers exists in units that have "Butt cells" configurations, which are rectangular photocells arranged end to end. This configuration enables the use of inexpensive standard photocells, and works very well for purely null sensing applications (detecting the beam at the exact center of the cells). However, if it is desired to detect when the laser beam is within a specific distance of the center of the cells, then this specific distance requires a position tolerance typically called the "dead band." Using the Butt cell configuration, the dead band will vary as the laser spot size changes and as the image energy distribution of the laser spot changes. These are very undesirable characteristics, since it is desirable for the dead band to be consistent for all types and sizes of laser beams.
Some of the other conventional photocell configurations use a series of ratiometric photocells (see the Petersen and Ake patents) that consist of two triangularly-shaped photocells arranged opposite each other, as in a tall rectangle with a diagonal line from corner-to-corner creating two rectangular regions (or their optical equivalents). These configurations can produce very consistent dead bands, however, they require a very large custom photocell which is typically very expensive to manufacture.